In known EUV radiation sources the radiation emission is generated through excitation of hot plasma from a source material, the plasma having emission lines in the EUV spectrum. For plasma generation, the source material must be excited inside a vacuum chamber from which the generated EUV radiation is then coupled out.
There are two established methods in the prior art for providing the source material in an EUV radiation source.
In a first method, the source material is provided in the form of individual droplets which cross through a plasma zone. In the plasma zone, individual droplets, as mass-limited target volume for excitation of a laser-generated plasma (LPP—Laser-Produced Plasma), are impinged by pulsed laser radiation. Such an EUV radiation source using an LPP is disclosed in the document WO 2008/027158 A2. There the radiation source has a vacuum chamber in which is arranged a feed device that can supply liquid source material either in droplet form or as a thin liquid column. Aside from metallic tin also tin bromides or tin hydride and tin alloys are used as liquid source materials.
In a second method, the source material is provided in the form of a thin layer on revolving elements, wherein the revolving element is at least partially immersed in a bath of the source material and transports the source material on its surface into a plasma zone in which the excitation of the source material takes place. This way of providing the source material also opens up the use of an LPP, with the generation of plasma being carried out directly through pulsed radiation of a laser beam focused on the surface of the revolving element, wherein the revolving element under the laser focus continuously provides fresh source material for the generation of an LPP.
On the other hand, revolving elements can also be used as electrodes facing one another for plasma generated through electric discharge (DPP—Discharge-Produced Plasma). In this case, the emitting plasma is generated through the discharge current between the electrodes. In this way of plasma generation, mostly a laser in the plasma zone is additionally directed to one of the revolving elements (electrodes) for local evaporation of the source material to prepare the source material in gaseous and pre-ionized form (cold plasma) for the discharge. Plasma generated in such a way is sometimes also called LDP plasma (LDP—Laser-Assisted Discharge-Produced Plasma). Such sources are described in detail, for instance, in the patent documents WO 05/101924 A1, U.S. Pat. No. 7,531,820 B2, U.S. Pat. No. 7,800,026 B2, U.S. Pat. No. 7,649,187 B2 and U.S. Pat. No. 8,040,033 B2.
In all cases described above the source material is impinged upon in the plasma zone by energy pulses to generate the EUV radiation-emitting plasma. A very small portion of the source material hit by the energy pulses (e.g. a droplet or a local area of a coating) is consumed through evaporation and ionization under this energy input, while the larger portion remains unused and falls downward through the force of gravity. To prevent contamination of the radiation source by evaporated and/or unused source material and so that the unused portion of the source material can be reused, arrangements are provided in the radiation source for catching the unused source material and for deflecting it into the immersion bath for electrode coating.
The only one of the above-named patent documents dealing in detail with catching unused source material is the aforementioned WO 2008/027158 A2. There a collecting receptacle is arranged below the plasma zone to catch the unused portion of the source material falling through the plasma zone. The collecting receptacle has a small opening at the top through which the source material falls into the collecting receptacle. The cross section of the opening is on the order of magnitude of the droplets or the thin column (jet) of source material. To facilitate collection of the source material, a negative pressure is generated in the collecting receptacle causing the unused portion of the source material to be sucked in through the upper opening of the collecting receptacle. Furthermore, the beam dump, which is necessary for the laser beam, is used to catch the unused portion of the source material that is projected from the plasma zone. Referring to the direction of the laser beam, the beam dump is arranged downstream of the source material falling through the plasma zone and primarily absorbs the unused laser radiation. To this end, aside from an opening directed to the laser beam, the beam dump has a cavity which has a funnel-shaped bottom and a discharge opening to a collecting receptacle so that the unused portion of the source material collected in the cavity can be removed. A negative pressure can be generated in the cavity to facilitate collection so that even very small, light particles of the unused source material can be caught.
The problem of catching unused source material is only described for LPP sources with a continuous (jet) or discontinuous (droplet) target beam, since such a beam of source material always transports more material through the plasma zone than can be used by the pulsed laser beam.
Regarding DPP sources with revolving electrodes, for instance in the above-mentioned U.S. Pat. No. 8,040,033 B2, it is assumed that only deflecting objects are needed which carry the source material back into the immersion baths for the continuous coating of the revolving electrodes. However, this solution involves the risk of a solidification of unused source material on the deflecting object followed by additional discharges or short circuits or repeated evaporation close to the plasma, causing the failure-free operation period of the radiation source to be extremely shortened.